I don't think Leyline Manipulator is as powerful as people seem to believe, and I doubt it will see much play this expansion for several reasons:

1.) Ethereal Peddler has never seen serious play, and tempo rogue has about as much early game card generation as tempo mage does right now. Tempo Mage doesn't even want to run Babbling Book, it is one of the weakest cards in the deck, and tempo mage usually prefers to hold on to Primordial Glyph instead of playing it turn 2. Arcanologist and Sorcerer's Apprentice are much better turn 2 plays. Therefore, this card isn't going to be much stronger turn 4 than a yeti because it isn't likely to discount much,

2.) This card doesn't allow for a better infinite damage combo than the combo in quest mage. You have to spend 10 mana to set the Simulacrum OTK up, and you can only do so when you have drawn most of your deck to find both copies of Simulacrum and a copy of Leyline Manipulator. Quest Mage is weak in the early game, but it doesn't have to waste 10 mana to get the combo pieces it needs. The ordering of the combo pieces drawn is also much trickier to manage for the non-quest exodia. Meanwhile, most of the time Quest Mage is using random spells defensively to survive, and Quest Mage can can play a partial combo to try and draw into its missing pieces. People have pointed out that the non-quest exodia can run extra early game defense and that it gets an extra card in the mulligan, but I don't think this is enough to save it. Because of the combo pieces needed for the OTK, and a need for extra combo pieces for consistency, there is really only room for 2 copies of Frostbolt in non-quest exodia lists that Quest Mage doesn't get.

3.) There isn't room for Leyline Manipulator in standard quest mage lists. Quest Mage doesn't have room for this card because it already runs 30 cards that it needs, and this can't replace any of them. Quest mage needs its combo pieces, card draw, freezes, and spell generation, and while a discount yeti would be helpful, it doesn't do enough to help get the combo online. Quest Mage would rather play frost bolt, and it doesn't even have room for it. You could try and play an elemental version of quest mage, but it would be much less efficient at getting the exodia combo online.

This expansion is over, Priest and Mage are going to destroy the show and make Gadgetzan's meta look fun. Whoever designed this card, Temporus, Psychic Scream, and Duskbringer have ZERO comprehension on what constitutes for fair game balance or simply healthy gameplay.

It's worth a mention it's 4 attack we have seen a few 4 attack minions that's the magic number priest don't like, we have also seen a few 1/1's and that's the magic stats evolve does not like, so under the hood of this set are some subtle "nerfs in air quotes" I think that was a clever way to handle it. I like the wording on this card you are gambling away your early game for a stronger mid game. I think it might be to strong if your bless RNG is strong, and very good if your Bless RNG is bad, so no real drawback to playing this in every mage deck.

Like I said, MOST of the time it's going to be spells, from the likes of babbling books, tomes, glyphs, spellstones, couriers, tempests, etc. But yeah It would also be good for minions like fireflys, pyros and simulacrum.